The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Heat tint is commonly observed in furnace treated stainless steel and in the heat affected zone of weldments, and is the result of the thickening of the naturally occurring transparent oxide layer on the surface of stainless steel. The colors formed are similar to the “temper colors” seen on other steel surfaces following heat treatment and range from pale straw hues to dark blue. As heat tints are formed on the surface of stainless steel, chromium is drawn to the surface due to its preferential susceptibility to oxidation relative to iron in the steel. This leaves a layer at and just below the surface with a lower chromium level than in the bulk of the steel, and so a surface with greatly diminished corrosion resistance. Other oxides present in the heat tint include ferrite that can initiate accelerated corrosion of the weld area.
Heat tint on stainless steel fabrications can be removed using acidic brush-on pastes or gels, spray pickling, or immersion tank pickling. Pickling is a metal surface treatment that removes impurities, such as rust or scale from metals. Most commonly, hydrofluoric acid is the base component that is used for the pickling of stainless steel. Unfortunately, products containing this very harmful and toxic acid require special handling procedures, and its fumes are dangerous to the user and environment.
Passivation is the use of a light coat of a protective material to create a barrier against corrosion. This is typically achieved by nitric acid, which is somewhat easier to handle and by itself is a less dangerous acid. For example, U.S. Pat. No. 6,844,304 to Lunner describes a pickling agent comprising nitric acid, urea, and a filler to remove an oxide layer of stainless steel after heat treatment. Nevertheless, and especially in combination with hydrofluoric acid, such pickling and passivation compositions are still problematic in use and disposal. In other attempts to remove heat tint, as described in U.S. Pat. No. 2,765,271 to Kreml, discoloration is removed by electrolytic treatment of the weld area using phosphoric acid. Similarly, as disclosed in WO 2013/036999 to Lewer, the heat tint is electrochemically removed using low voltage high amperage current with an electrolytic fluid comprising a potassium phosphate salt of an acid at neutral pH. While such compositions are generally benign to the environment and operator, use is complicated by the need for specific equipment and electricity.
In still further known methods, chemically inert solid materials in a carrier fluid are used in a typically high pressure spray gun to remove weld marks from stainless steel as described in EP 2801443. However, such methods will require at least some additional equipment to provide motive force to the cleaning composition.
Therefore, even though various compositions and methods for removing weld marks and increasing corrosion resistance of stainless steel are known in the art, there is still a need to provide improved compositions and methods for treatment of stainless steel.